FIG. 1 shows a conventional vertical light emitting diode (LED), which includes a sandwich structure formed by a first-type semiconductor layer 1A, an active layer 2A and a second-type semiconductor layer 3A. Below the second-type semiconductor layer 3A, a substrate 4A and a second-type electrode 5A are sequentially formed. A first-type electrode 6A is disposed at a surface of the first-type semiconductor layer 1A. The first-type semiconductor layer 1A and the second-type semiconductor layer 3A are a combination of one and the other of a P-type semiconductor and an N-type semiconductor.
By applying a forward bias between the first-type electrode 6A and the second-type electrode 5A, the first-type semiconductor layer 1A and the second-type semiconductor layer 3A provide electrons and holes, respectively. The electrons and holes are combined in the active layer 2A to perform energy level jump to further generate an excited light having a constant wavelength.
FIG. 2 shows a conventional horizontal LED, which similarly includes a sandwich structure formed by a first-type semiconductor layer 1B, an active layer 2B and a second-type semiconductor layer 3B. The first-type semiconductor layer 1B is formed on a substrate 4A, and a second-type electrode 5B and a first-type electrode 6B are respectively disposed at same sides of the second-type semiconductor layer 3B and the first-type semiconductor layer 1B. By applying a voltage between the first-type electrode 6B and the second-type electrode 5B, electrons and holes are combined in the active layer 2B to generate an excited light.
Referring to FIG. 3, the U.S. Pat. No. 7,223,998, “White, single or multi-color light emitting diodes by recycling guided modes”, discloses a photon recycling LED that mainly includes a nitride LED 9. On the nitride LED 9, a non-doped semiconductor layer 9A, a nitride photoluminescent active layer 9B and another non-doped semiconductor layer 9A are sequentially stacked. An electroluminescent light 9C generated by the nitride LED 9 enters the nitride photoluminescent active layer 9B to generate a photoluminescent light 9D having another wavelength. In other words, the presence of the nitride photoluminescent active layer 9B is to generate a photoluminescent light having another wavelength, i.e., lights of multiple colors are allowed to be blended to generate a white light with a preferred color rendering index (CRI).
Any of the vertical LED, the horizontal LED and another type of LED is capable of generating and emitting light having different wavelengths given that an appropriate material is selected as an active layer according to actual requirements. The light generated includes visible light in various colors, and high energy invisible light such as ultraviolet light.
High energy invisible light, such as ultraviolet light, extensively applied in public environment applications, is capable of effectively killing viruses and bacteria or reducing activity levels of viruses and bacteria to prevent massive contagions.
As high energy invisible light cannot be perceived by human eyes, human bodies can be easily exposed to high energy invisible light without knowing. Pathological changes of the skin or eyes may be caused by an excessive accumulated dosage of high energy invisible light, thus endangering lives and health of users.